The present invention relates to methods of making microelectronic assemblies, and to components for use in fabrication of microelectronic assemblies.
Complex microelectronic devices such as modern semiconductor chips require numerous connections to other electronic components. For example, a complex microprocessor chip may require hundreds of connections to external devices.
As described in certain preferred embodiments of commonly assigned U.S. Pat. Nos. 5,148,265 and 5,148,266, a chip package may include a flexible sheetlike structure referred to as an xe2x80x9cinterposerxe2x80x9d or xe2x80x9cchip carrierxe2x80x9d having terminals disposed on a flexible sheetlike structure. The interposer may be disposed on the front or contact bearing surface of the chip so that the terminals face away from the chip. The terminals are connected to contacts on the chip by flexible leads. Preferably, a compliant layer is disposed between the terminals and the chip. In certain preferred embodiments, the packaged chip occupies the same area, or only a slightly larger area than the chip itself. The packaged chip can be readily tested and can be mounted to a substrate such as a printed circuit board by bonding the terminals of the chip carrier to the contact pads of the substrate. In use, the terminals on the chip carrier are free to move relative to the chip. This allows the assembly to compensate for differential thermal expansion and warpage of the chip or substrate without imposing significant stresses on the bonds.
As taught in certain embodiments of commonly assigned U.S. Pat. No. 5,518,964, microelectronic assemblies incorporating flexible leads can be fabricated using a first element or connection component including a dielectric structure having leads on a bottom surface. Each such lead has a fixed end permanently attached to the dielectric structure and, typically, connected to one or more terminals on the top surface of the dielectric structure. Each such lead also has a free or tip end remote from the fixed or terminal end. Preferably, the free or tip ends of the leads are releasably attached to the dielectric structure. This element can be juxtaposed with a second microelectronic element such as a semiconductor chip or wafer, and the free ends of the leads may be bonded to contacts on such second microelectronic element. After bonding, the elements are moved vertically away from one another through a predetermined displacement, thereby detaching the free or tip ends of the leads from the bottom surface of the dielectric component and deforming the leads to a vertically extensive configuration. Preferably, a curable encapsulant is introduced between the elements and around the leads during or after the moving step, so as to provide a compliant layer between the dielectric layer and the second microelectronic element. This arrangement allows fabrication of compliant chip assemblies having advantages similar to those discussed above with respect to the ""265 and ""266 patents using a process which permits simultaneous connection and forming of numerous leads. In certain preferred embodiments according to the ""964 patent, one of the microelectronic assemblies may include numerous semiconductor chips. For example, one microelectronic element may be a wafer incorporating numerous chips, and leads on all of the chips may be connected and formed in the same operations. After these operations, the resulting large assembly can be severed to form individual units each including one or more of the chips originally present in the wafer, together with a portion of the dielectric element and the terminals thereon.
In those embodiments of the ""964 invention which use a pre-formed connection component with leads thereon, and which register the connection component with a wafer or other microelectronic device, the spacing between the leads on the component desirably is controlled precisely. This allows registration of the free ends of the leads with contacts on a wafer or other microelectronic device. For example, certain preferred embodiments disclosed in the ""964 patent use a temporary reinforcing layer overlying the dielectric layer, and also use a rigid, ring-like frame to maintain the dielectric layer and the reinforcing layer to maintain the dielectric layer in tension. These features help to control thermal expansion and contraction of the connection component during the processes used to bond the free ends of the leads to the wafer or other microelectronic component, and help to maintain the desired spacing between the lead free ends.
Commonly assigned U.S. Pat. No. 5,989,936 discloses enhancements to processes taught in the ""964 patent. Certain processes taught in the ""936 patent use a metallic conductive element. Leads are provided on a bottom surface of such element. Each lead has a fixed end permanently attached to the conductive element, and a free or tip end releasably attached to the conductive element. The conductive element with the leads thereon is juxtaposed with a microelectronic element such as a chip or wafer, and the tip ends of the leads are bonded to the contacts of the microelectronic element. The conductive element and microelectronic element are moved away from one another, thereby bending the leads into a vertically-extensive configuration, and a flowable material is injected and cured so as to form a dielectric layer around the leads. After formation of the dielectric layer, the conductive layer is etched or otherwise removed so as to leave portions of the conductive layer as individual terminals associated with the individual leads. In a variant of this process, each lead may have a pre-formed terminal of a first metal such as gold, provided at the juncture of the fixed end of the lead and the conductive layer before the lead-bonding and moving steps. The conductive layer is formed from a second metal, such as copper. When the conductive layer is etched away, the pre-formed terminals remain.
Preferred arrangements taught in commonly assigned U.S. Pat. Nos. 5,859,472 and 6,191,368 disclose curved lead configurations useful in the foregoing processes. Commonly assigned U.S. Pat. No. 5,976,913 discloses further improvements in such processes. In the preferred arrangements taught in this application, restraining straps which are shorter and stronger than the leads constrain the motion of the elements as they move away from one another to deform the leads. The preferred aspects of commonly assigned U.S. Pat. No. 5,798,286 disclose further improvements in the basic process taught in the ""964 patent, with respect to the processing of arrays of individual semiconductor chips bonded to leads on a single dielectric element so that the leads associated with all of the separate chips can be deformed and in a single operation.
The disclosures of all of the aforementioned patents and applications are incorporated herein by reference.
Despite these advances in the art of making microelectronic assemblies, still further improvements would be desirable.
One aspect of the present invention provides methods of making microelectronic assemblies. A method in accordance with this aspect of the invention includes the step of providing a sheet including a main portion and a plurality of gaps extending through the sheet. These gaps partially surround and define a set of elongated lead regions in the sheet. Each lead region has a fixed end connected to the main portion of the sheet and a tip end remote from the fixed end. The lead regions of the sheet include conductors extending between the tip ends and the fixed ends. In a further step of the method, the sheet is juxtaposed with a microelectronic component so that the tip ends of the lead regions on the sheet are aligned with contacts on the microelectronic element. The tip ends of the lead regions are connected to contacts on the microelectronic element. After the tips ends of the lead regions have been attached to the contacts, the main portion of the sheet and the microelectronic element are moved away from one another through a predetermined displacement in a vertical direction as to bend the lead regions downwardly from the main portion and thereby form leads projecting from the main portion of the sheet downwardly to the microelectronic element.
Desirably, the step of providing the sheet includes the step of bonding the sheet to a reinforcing structure prior to the step of connecting the tip ends of the leads to the contacts. The sheet may be bonded to the reinforcing structure or support structure before the gaps in the sheet are formed. In a particularly preferred arrangement, the sheet includes a polymeric dielectric layer such as a polyimide layer and the step of providing the sheet includes the step of providing conductive metallic strips on a surface of the polymeric layer. For example, some or all of the strips may be formed on a bottom surface of the polymeric layer facing away from the reinforcing structure. The step of forming the gaps in the sheet may include the step of etching or plating the dielectric as, for example, by plasma etching or laser ablating the dielectric layer. The metallic strips may protect regions of the dielectric layer underlying the strips during the gap-forming process. Typically, the sheet is provided with terminals facing upwardly, toward the reinforcing structure, the terminals being connected to the conductors of the lead regions. The reinforcing structure is removed near the end of the process, leaving the resulting assembly with exposed terminals connected by the flexible leads to the contacts on the microelectronic element.
As further discussed below, the sheet can be fabricated in a few simple, inexpensive steps. Moreover, the formed leads can incorporate polymeric portions derived from the sheet. The polymeric layers in the leads physically reinforce the metallic conductors, and permit the use of thin metallic conductors. These features enhance the fatigue resistance of the leads.
As the lead regions of the sheet are bent downwardly, out of the plane of the main portion of the sheet, openings are left in the spaces formerly occupied. Preferably, the process also includes the step of injecting a flowable material between the main portion of the sheet and the microelectronic element, so that the flowable material encapsulates the projecting leads, and then curing this flowable material to form a dielectric layer, preferably a compliant layer such as a gel or elastomer. The reinforcing structure blocks the flow of the flowable material through the openings in the main portion of the sheet. This prevents the encapsulant from covering over the terminals.
Desirably, the step of removing the reinforcing structure is performed after the tip ends of the leads are attached to the contacts on the microelectronic element and, most preferably after the flowable dielectric material is introduced between the main portion of the sheet and the dielectric element. The reinforcing structure stabilizes the dimensions of the sheet and allows accurate alignment of the tip ends of the leads with the contacts on the microelectronic element. Most preferably, the reinforcing structure is formed from a material having uniform, predictable thermal expansion and contraction properties, such as a metal, a glass or certain nonpolymeric material. In a particularly preferred arrangement, the coefficient of thermal expansion of the reinforcing structure is approximately equal to that of the microelectronic element. For example, where the microelectronic element is a wafer or other structure formed principally from silicon, the reinforcing structure desirably has a coefficient thermal expansion between about 2 and about 6xc3x9710xe2x88x926/xc2x0 C. This adds in maintaining registration between the tips of the lead regions and the contacts on the microelectronic element over the main temperatures encountered during the assembly process.
Preferably, the top surface of the sheet, facing toward the reinforcing element adheres to the reinforcing element. Depending upon the fabrication methods utilized, this adhesion may result from a direct bond between the top surface of the sheet and the reinforcing element or from an intervening adhesive layer. During the moving step, as the lead regions are bent downwardly, each lead region may be peeled away from the reinforcing layer. The reinforcing structure may be removed by etching, or else may be peeled away from the main portion of the sheet, typically after the moving step. Alternatively, the bond between the main portion of the sheet and the reinforcing layer may be broken by degrading the adhesive through the application of heat or radiant energy. In a further variant, the bond between the lead regions and the reinforcing structure may be broken selectively in the lead regions, as by selective application of heat or, preferably, by selective application of radiant energy to the lead regions. The reinforcing structure may be transparent to radiation within a predetermined wavelength band as, for example, ultraviolet radiation. As discussed below, such radiation can be used to release adhesive bonds between the reinforcing structure and the sheet.
The conductors on the lead regions desirably include metallic strips extending along one or both surfaces of the sheet in the lead regions. In one arrangement, the metallic strips extend along bottom surfaces of the lead regions and extend to terminals disposed in the main region of the sheet on the bottom surface of the sheet, i.e., the surface of the sheet facing away from reinforcing structure. The sheet desirably has openings or xe2x80x9cviasxe2x80x9d extending through it at the terminals, so that the terminals are exposed to the top surface of the sheet. Electrically-conductive bonding materials such as eutectic bonding alloys, solder, diffusion bonding alloys and electrically-conductive polymeric compositions may be disposed on the tip ends of the leads in contact with the metallic strips. In this arrangement, only one metallic layer need be formed on the sheet. In a variant of this approach, the main region of the sheet may be provided with an electrically-conductive potential reference plane formed from the same metallic layer used to form the metallic strips on the bottom surfaces of the lead regions. In other embodiments, conductive elements such as metallic strips are provided on both surfaces of lead regions. For example, an electrically-conductive potential reference plane may be formed in the main region of the sheet on the top surface of the sheet, and the lead regions may be provided with reference conductors on the top surface continuous with the potential plane layer. Signal conductors may be provided on the bottom surfaces of the lead regions. In other arrangements, the bottom surfaces of the lead regions may be provided with reference conductors, whereas the signal conductors may be provided on the top surfaces of the lead regions. In still other arrangements, paired signal conductors may be provided on the top and bottom surfaces of the lead regions and both conductors of each such pair may be conducted to contacts of microelectronic elements.
Yet another aspect of the invention provides components for forming microelectronic assemblies. Components in accordance with this aspect of the invention desirably include a sheet having a main portion and a plurality of gaps partially surrounding and defining a set of flexible, elongated lead regions within the sheet. Each such lead region has a fixed end connected to the main portion of the sheet and a tip end remote from the fixed end. As discussed above in connection with the methods, the lead regions of the sheet include conductors extending between the tip ends and the fixed ends of the lead regions. Desirably, the sheet includes a continuous layer of a polymeric material having oppositely facing top and bottom surfaces extending within the main portion of the sheet and within the lead regions. The sheet typically includes metallic strips extending on the polymeric layer in the lead regions. Some or all of these strips desirably extend from the lead regions onto the main region of the sheet. At least some of the metallic strips are electrically isolated from others of the metallic strips, so that the strips may serve as signal conductors in the finished assemblies. As discussed above in connection with the methods, the metallic strips may be provided on the top surface; on the bottom surface; or on both surfaces of the sheet in the lead regions. Desirably, the component further includes terminals attached to the sheet in the main region and exposed to the top surface of the sheet, the terminals being electrically connected to the strips. For example, where the terminals are physically disposed on the bottom surface of the strip, vias may be provided through the sheet in registration with the terminals. The vias may have metallic, conductive liners extending to the top surface. Alternatively, the vias may be left unfilled. The gaps defining straight lead regions may be substantially U-shaped, with the bases of the U-shapes at the top ends of the leads. Alternatively, the lead regions may be curved, and the gaps bounding each lead region may be correspondingly curved. The sheet may be provided with features which reinforce it against teasing at the ends of the gaps where the lead regions join the main region of the sheet. For example, the sheet may have holes slightly larger in diameter than the width of a gap disposed at the ends of the gap.
The component may also include a reinforcing element, such as a reinforcing element having controlled, predictable thermal expansion characteristics as discussed above. Desirably, the top surface of the sheet is releasably bonded to the bottom surface of the reinforcing element.
Still further aspects of the present invention include methods of making components such as the components discussed above. Methods according to this aspect of the invention desirably include the step of providing a polymeric layer and forming metallic strips on the polymeric layer. The method desirably further includes the step of forming gaps in the polymeric layer by selectively removing portions of such layer so as to form gaps extending partially around lead regions of the polymeric layer and so as to leave a main region of the polymeric layer, physically connected to the lead regions. The steps of forming the metallic strips and forming the gaps are performed so as to leave one or more strips extending onto each lead region from the main region. Desirably, the metallic strips are formed prior to the gap-forming step, and the strips protect the polymeric layer during the gap-forming step. The method of making a component may further include the step of providing a reinforcing element bonded to the top surface of the polymeric layer. Desirably, the step of providing the reinforcing element is performed prior to the gap-forming step, so that the reinforcing element holds the various portions of the layer during and after the gap-forming step.
Still further aspects of the invention include finished assemblies. As discussed below, the finished assemblies may include sheetlike elements with polymeric layers and with an array of terminals thereon, the terminals being connected to a chip or other microelectronic element by leads including polymeric layers integral with the polymeric layer of the sheetlike element and metallic conductive strips.
These and other objects, features and advantages of the present invention would be more readily apparent from the detailed description of the preferred embodiments set forth below, taken in conjunction with the accompanying drawings.